Java条件语句（足够简单）

Question

My program is to tell if a year entered is a leap year that meets these requirements:

A year is a leap year if it is divisible by 4. But if the year is divisible by 100, it is a leap year only when it is also divisible by 400.

Create a program that checks whether the given year is a leap year.

Here are what results should be produced per input:

Type a year: 2011 The year is not a leap year.

Type a year: 2012 The year is a leap year.

Type a year: 1800 The year is not a leap year.

Type a year: 2000 The year is a leap year.

This is what I came up with:

import java.util.Scanner;

public class LeapYear {

    public static void main(String[] args) {
        Scanner reader = new Scanner(System.in);

        System.out.println(" Type a year ");
        int number = Integer.parseInt(reader.nextLine());

        if (number % 4 == 0 ) {
            System.out.println(" This is a leap year");
        } else if (number % 100 == 0 && number % 400 == 0) {
            System.out.println(" This is a leap year ");
        } else {
            System.out.println( "This is not a leap year");
        } 
    }
}  

All of them work except 1800. 1800 is not a leap year and my program says that it is (it is not divisible by 400). It seems that (number % 100 == 0 && number % 400 == 0) only works if (number % 4 == 0 ) is not there. Why does my program not work correctly?

Answer 1

Consider what happens when the year 100 is inputted. 100 % 4 ==0 is TRUE , so your code outputs that the year is a leap year. The problem is, 100 % 100 == 0 is also TRUE , but your code will never reach this line, nor will it check if 100 % 400 == 0 is TRUE .

You printed out the result before you checked all of the conditions!

Change the structure of your if else.

This sounds like a homework assignment, so I don't want to give you the answer. You should have all of the information needed to reach it from here. If not, feel free to comment with any questions.

EDIT: Since you appear to have gotten the core of the problem, here is what is wrong with your answer.

Your brackets { and } are in the wrong places, among other things. Placing your brackets in the locations I do is part of the Java standard, and makes your code easier to read, understand, and debug. (It makes it easier to identify when they are missing as well.)

Your code should look something like this:

// Pay attention to a few things here. It checks if it is divisible by 4
// since every leap year must be divisible by 4. If it is,
// it checks if it is divisible by 100. If it is, it must also
// be divisible by 400, or it is not a leap year. So, if it is divisible
// by 100 and NOT divisible by 400, it is not a leap year. If it isn't 
// divisible by 100, control flows to the else statement, and since we
// already tested number % 4 we know it is a leap year.
// Pay special attention to where I located my { and }, this is the 
// standard way to do it in java, it makes your code readable by others.

if(number % 4 == 0) {
    if((number % 100 == 0) && !(number % 400 == 0)) { // NOTE THE ! OPERATOR HERE
        System.out.println("The year is NOT a leap year.");
    } else {
        System.our.println("The year is a leap year.");
    }
} else {
    System.out.println("The year is NOT a leap year");
} 

Answer 2

Try this

bool isLeap = false;
if (number % 4 == 0) {
  isLeap = true;
  if (number % 100 == 0)
    isLeap = false;
  if (number % 400 == 0)
    isLeap = true;
}
if (isLeap) { //print stuff

That should work for you. You're preempting your own logic by outputting early.

Answer 3

    import java.util.Scanner;

    public class LeapYear {

    public static void main(String[] args) {
    Scanner reader = new Scanner(System.in);

    System.out.println(" Type a year ");
    int number = Integer.parseInt(reader.nextLine());

    if (number % 4 == 0 ) 
        if (number % 100 == 0) 
          if (number % 400 == 0)
          {
        System.out.println(" This is a leap year");
    }   else {
        System.out.println( "This is not a leap year");
    } 
}

}

Here you go HC. I only cannot get it to print properly. Close?